Frequency standards are devices that produce a signal whose frequency displays extremely small fluctuations over the long term. These fluctuations are called instability, generally quantified as a fractional deviation or fractional variance. One of the chief goals in the development of frequency standards is to reduce instability and so increase the precision of the output frequency, with the best current standards achieving fractional instability ≈10−18. Clocks are a derivative technology of frequency standards, which are based on some frequency standard, whose stability is a measure of the frequency deviation of its output ‘tick’ signal.
Frequency can be more precisely measured than other physical quantities and therefore, frequency standards serve as the basis for the definitions of other physical units. One application of frequency standards is an atomic clock, which maintains international time. High-stability frequency standards are also used for the operation of global positioning systems and for geodesy, as well as laboratory tests of fundamental physics. Frequency standards are classified into active and passive types.
Active frequency standards are self-sustaining oscillators whose very high stability is generally quantified by their quality factor. Such standards are often also called ‘open-loop’ and perform best on relatively short timescales (10-100 s). State-of-the-art frequency standards achieve extremely low instability through a passive configuration. In general, passive standards such as atomic beam and fountain clocks, trapped ion clocks and optical lattice clocks, achieve better long-term stability than active standards [1].
A major limit to the performance of passive frequency standards comes from the quality of the local oscillator (LO) used to interrogate the atomic transition. The LO frequency evolves randomly in time due to intrinsic instabilities from the underlying hardware, leading to deviations of the LO frequency from that of the stable atomic reference.
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